Column Effective Length

Setting	Description
BS5950:1990	There are numerous parameters that can affect the effective length of a member subject to axial load. The criteria are set in the dialog shown by selecting Criteria-Effective Length, or on a member-by-member basis using the Assign menu. For columns, the engineer can specify an appropriate effective length factor per Table 24 in BS 5950. Otherwise, the engineer can specify that the effective length be calculated according to the requirements of Appendix E. This appendix considers the members framing into the column, and whether a section is sway-sensitive or non-sway (See Post-Processing Criteria above for making this designation) to calculate the effective length of the column. The equations used to calculate the Le/L factor are taken from Annex E in the Draft Amendments for BS 5950, dated April 1998. The structure is assumed to be comprised of rigid joints where moment connected, and is either fully braced against sidesway (k3=infinity) or has unrestricted sidesway (k3=0), no partial lateral support is considered. The effective length factor used to calculate the axial capacity of the column is also used to calculate the lateral torsional buckling capacity for the beam when subject to bending. The following assumptions are made in implementing Appendix E: A column pinned top and bottom (or where all beams and columns framing into the column in question are pinned) will be assigned an effective length factor of 1.0. This is true even when the column is indicated as being subject to sidesway. A column fixed at the foundation will be assigned a k2 value of 0.5 per BS 5950, Clause 5.1.2.4. (a). A column pinned at the foundation will be assigned a k2 based on a base stiffness equal to 10% of the column itself. This is according to BS 5950, Clause 5.1.2.4. (b) and results in a k factor of 0.909. Any other columns, which are continuous but have no rigid connected beams framing into the joint will be assigned a k value of 0.909, similar to the factor at a pin support per BS 5950 Clause 5.1.2.4. A beam designated as composite is assumed to carry a concrete slab and the relevant beam stiffness kb is taken equal to I/L. Per Appendix E.4.1, the beam stiffness for non-composite beams is taken as 0.5I/L if braced against sidesway and as 1.5I/L if not braced against sidesway. Knee brace columns are assigned a Le /L value of 1.0 in the direction of the knee brace. The effective length factor for brace members subject to axial load is provided by the engineer for all shapes except double angles. For double angles, the connection detail must be provided and the effective length factor is then determined according to Clause 4.7.10.3 of BS 5950. Single angles are not currently accommodated in the structural system. Capacity of T-Section struts is computed per section 4.7.10. As described in table 4.15 the capacity of the T-Section is affected by the connection configuration. To specify the appropriate connection type for the T-Section select the Criteria-Effective Length Command and choose the appropriate option in the dialog that appears as shown below The Le/L factor used to calculate the axial capacity is also used in the calculation of bending capacity lateral torsional buckling where a brace is subject to end moments. The compression capacity of I-section and RHS beams subject to axial load is calculated using the effective length factors provided by the engineer. For channels, the end connection details are used to determine the effective length factor according to Section 4.7.10.4 of BS 5950. Double channels are not currently accommodated in the structural system. In calculating the bending capacity of an unbraced beam the effect of end fixity on the lateral torsional buckling (LTB) capacity of the section is considered. Loads are assumed to be normal loads (applied at the shear center). For a segment continuous through brace points, the Le is equal to the unbraced length. Where a beam is continuous through a column or supporting girder an effective length factor of 0.7 is assigned to that end of the segment. This is indicative of the column providing the beam a measure of restraint for bending about the minor axis. Where a beam is pinned at a support, or where a cantilever tip occurs, the effective length factor is taken as the value provided by the engineer in the Criteria-Effective Length dialog. The effective length factor for an unbraced segment is taken as the average of the factors at the segment ends (Refer to Table 10). For I and RHS struts the minor axis effective length factor can be the same as the effective length calculated for the segment for lateral torsional buckling if the engineer indicates as such in the Effective Length dialog described previously.
BS5950:2000	For design purposes the engineer can select to apply the column effective length factors in the Criteria-Effective Length dialog (See BS5950-1:1990 discussion above), or specify a specific value to be used. The engineer may specify that the effective length ratio LE/L either be calculated per Annex E or that it be set to a specific value. Per Annex E the LE/L ratio will be calculated based on whether the column is non-sway or sway-sensitive. This is true in all cases except as described in the section on moment amplification below. Refer to the BS5950-1:1990 discussion of Effective length for more detailed description of the calculation of the effective length factor. The method used by the program to calculate the columns effective length factor is as depicted in Figure 3 2. The following discussion regarding effective length for columns assumes that the columns axis sway state is known. That is, the program knows if a column is sway-sensitive or non-sway in a particular axis. Refer to section 3.2.2.3 for discussion on the calculation of this state. For a non-sway axis the program will consider if the user has designated a specific effective length value or indicated to use Annex E to calculate the effective length. In the case of using Annex E the program will use the No-Sway Annex E formulations. For a Sway-Sensitive column axis the program first determines if P-Delta has been performed. If P-Delta is performed the program will either use the user specified effective length factor (if assigned) or will use the No-Sway Annex E effective length factors per 5.6.4(b). The No-Sway effective length factors are used as in this case it is assumed that the amplified sway approach (2.4.2.7b) is being applied and the 'kAMP x Ms' values will be replaced by the sway moments from the P-Delta Analysis (refer to Moment Amplification (BS5950-1:2000) ). If P-Delta is not performed the program will respond based on one additional criteria the user can assign. The user can designate to either use the effective length factor per 2.4.2.7a or the amplified sway method per 2.4.2.7b (refer to 2.6.6 Sway-Sensitive Design Method (BS5950:2000 Only) ). If the effective length approach is selected the program will use the user specified effective length factor or the Sway-Sensitive Annex E Formulations per 5.6.4(a)/2.4.2.7(a). If the amplified sway approach is selected the program will use the user specified effective length factor or the Non-Sway Annex E Formulations per 5.6.4(b)/2.4.2.7(b). The logic used to calculate the effective length factor for columns is illustrated in the manual.

Setting

Description

BS5950:1990

There are numerous parameters that can affect the effective length of a member subject to axial load. The criteria are set in the dialog shown by selecting Criteria-Effective Length, or on a member-by-member basis using the Assign menu.

For columns, the engineer can specify an appropriate effective length factor per Table 24 in BS 5950. Otherwise, the engineer can specify that the effective length be calculated according to the requirements of Appendix E. This appendix considers the members framing into the column, and whether a section is sway-sensitive or non-sway (See Post-Processing Criteria above for making this designation) to calculate the effective length of the column. The equations used to calculate the Le/L factor are taken from Annex E in the Draft Amendments for BS 5950, dated April 1998. The structure is assumed to be comprised of rigid joints where moment connected, and is either fully braced against sidesway (k3=infinity) or has unrestricted sidesway (k3=0), no partial lateral support is considered. The effective length factor used to calculate the axial capacity of the column is also used to calculate the lateral torsional buckling capacity for the beam when subject to bending. The following assumptions are made in implementing Appendix E:

A column pinned top and bottom (or where all beams and columns framing into the column in question are pinned) will be assigned an effective length factor of 1.0. This is true even when the column is indicated as being subject to sidesway.

A column fixed at the foundation will be assigned a k2 value of 0.5 per BS 5950, Clause 5.1.2.4. (a).
A column pinned at the foundation will be assigned a k2 based on a base stiffness equal to 10% of the column itself. This is according to BS 5950, Clause 5.1.2.4. (b) and results in a k factor of 0.909.
Any other columns, which are continuous but have no rigid connected beams framing into the joint will be assigned a k value of 0.909, similar to the factor at a pin support per BS 5950 Clause 5.1.2.4.
A beam designated as composite is assumed to carry a concrete slab and the relevant beam stiffness kb is taken equal to I/L. Per Appendix E.4.1, the beam stiffness for non-composite beams is taken as 0.5I/L if braced against sidesway and as 1.5I/L if not braced against sidesway.

Knee brace columns are assigned a Le /L value of 1.0 in the direction of the knee brace.

The effective length factor for brace members subject to axial load is provided by the engineer for all shapes except double angles. For double angles, the connection detail must be provided and the effective length factor is then determined according to Clause 4.7.10.3 of BS 5950. Single angles are not currently accommodated in the structural system. Capacity of T-Section struts is computed per section 4.7.10. As described in table 4.15 the capacity of the T-Section is affected by the connection configuration. To specify the appropriate connection type for the T-Section select the Criteria-Effective Length Command and choose the appropriate option in the dialog that appears as shown below The Le/L factor used to calculate the axial capacity is also used in the calculation of bending capacity lateral torsional buckling where a brace is subject to end moments.

The compression capacity of I-section and RHS beams subject to axial load is calculated using the effective length factors provided by the engineer. For channels, the end connection details are used to determine the effective length factor according to Section 4.7.10.4 of BS 5950. Double channels are not currently accommodated in the structural system. In calculating the bending capacity of an unbraced beam the effect of end fixity on the lateral torsional buckling (LTB) capacity of the section is considered. Loads are assumed to be normal loads (applied at the shear center). For a segment continuous through brace points, the Le is equal to the unbraced length. Where a beam is continuous through a column or supporting girder an effective length factor of 0.7 is assigned to that end of the segment. This is indicative of the column providing the beam a measure of restraint for bending about the minor axis. Where a beam is pinned at a support, or where a cantilever tip occurs, the effective length factor is taken as the value provided by the engineer in the Criteria-Effective Length dialog. The effective length factor for an unbraced segment is taken as the average of the factors at the segment ends (Refer to Table 10). For I and RHS struts the minor axis effective length factor can be the same as the effective length calculated for the segment for lateral torsional buckling if the engineer indicates as such in the Effective Length dialog described previously.

BS5950:2000

For design purposes the engineer can select to apply the column effective length factors in the Criteria-Effective Length dialog (See BS5950-1:1990 discussion above), or specify a specific value to be used. The engineer may specify that the effective length ratio LE/L either be calculated per Annex E or that it be set to a specific value. Per Annex E the LE/L ratio will be calculated based on whether the column is non-sway or sway-sensitive. This is true in all cases except as described in the section on moment amplification below. Refer to the BS5950-1:1990 discussion of Effective length for more detailed description of the calculation of the effective length factor. The method used by the program to calculate the columns effective length factor is as depicted in Figure 3 2.

The following discussion regarding effective length for columns assumes that the columns axis sway state is known. That is, the program knows if a column is sway-sensitive or non-sway in a particular axis. Refer to section 3.2.2.3 for discussion on the calculation of this state.

For a non-sway axis the program will consider if the user has designated a specific effective length value or indicated to use Annex E to calculate the effective length. In the case of using Annex E the program will use the No-Sway Annex E formulations.

For a Sway-Sensitive column axis the program first determines if P-Delta has been performed.

If P-Delta is performed the program will either use the user specified effective length factor (if assigned) or will use the No-Sway Annex E effective length factors per 5.6.4(b). The No-Sway effective length factors are used as in this case it is assumed that the amplified sway approach (2.4.2.7b) is being applied and the 'kAMP x Ms' values will be replaced by the sway moments from the P-Delta Analysis (refer to Moment Amplification (BS5950-1:2000) ).
If P-Delta is not performed the program will respond based on one additional criteria the user can assign. The user can designate to either use the effective length factor per 2.4.2.7a or the amplified sway method per 2.4.2.7b (refer to 2.6.6 Sway-Sensitive Design Method (BS5950:2000 Only) ). If the effective length approach is selected the program will use the user specified effective length factor or the Sway-Sensitive Annex E Formulations per 5.6.4(a)/2.4.2.7(a). If the amplified sway approach is selected the program will use the user specified effective length factor or the Non-Sway Annex E Formulations per 5.6.4(b)/2.4.2.7(b).

The logic used to calculate the effective length factor for columns is illustrated in the manual.